Radiotherapy has been used to treat cancer in the human body since early 1900. Even though radiation of cancer tumours is known to be efficient, mortality rate for many cancers remained virtually unchanged for a long time. The major reasons for this have been the inability to control the primary tumour or the occurrence of metastases. Only by improving the local control may the treatment be more effective.
In the last years Treatment Planning Systems, TPS, in Radiation Therapy have developed extensively and are now able to take into account the anatomy of the specific patient, and in a time efficient way plan a more optimised treatment for each individual patient including a homogenous dose to the target and a minimum dose to risk-organs.
The treatment technique to deliver this optimised treatment is more complicated than conventional treatments because each field must be modulated laterally in intensity and thereby compensate for the patients contour and anatomic heterogeneity. The technique is called IMRT—Intensity Modulated Radiation Therapy. The delivery can be done using compensators, i.e., filters individually made for each projection, that reduce the intensity to a predefined level in each part of the field due to attenuation of the primary photon beam. However when using several projections (4-8), each projection requiring individual compensators, this technique is time-consuming and requires a lot of effort. Additionally the attenuation of the beam in the filter causes an unwanted change of the beam's spectral distribution, thereby complicating the whole process. The most common way to deliver IMRT is therefore using the MLC (Multi Leaf Collimator) a device that consists of thin blocks (collimator-leafs) that can be individually positioned to block a small part of the field and thereby shape the beam in the lateral direction into various irregular shapes. In each projection, the collimator-leafs are moved during the treatment and thereby various parts of the cross-section of the beam are irradiated during various times, i.e., the dose distribution is modulated. The conformity of the dose distribution to the tumour can be further improved using even more sophisticated techniques also changing the projection while the beam is on, e.g., ARC-therapy.
In conventional therapy it is sufficient to make periodic verification on the level of the dose distribution on the central axis and in a few points off-axis to verify the beam-symmetry and beam-flatness. The new treatment technique is complicated and involves the transfer of information between several systems and therapy system sub-modules, and the cross section of the beam is individualized for each projection on each patient, thereby extended quality assurance is required.
The fundamental function of IMRT, of building-up the dose in the field by blocking some parts of it longer than others often increases the beam-on time, and thereby the dose to the area outside the field increases. In IMRT, accurate measurement of the dose to the areas outside the field is thereby more important than in conventional treatment, a requirement that further increases the demands in the measurement process.
Good quality control procedures in a radiation therapy clinic treating with IMRT technique include:                Machine specific quality assurance, e.g., stability check of the dose rate, time for the treatment system to stabilise, mechanical QA of the MLC etc. before the treatment machine is accepted to be used for treatments.        Pre treatment verification—Measurement performed on each individual treatment plan, before the patient is given the first treatment fraction, to verify the ability to deliver the treatment accurately.        Patient dosimetry or in vivo dosimetry—Verification of the delivered dose to the patient during the actual treatment, see the Swedish patent application 0201371-2.        
Pre treatment verification can be done for each individual projection using a 2D detector in a flat phantom positioned perpendicular to the beam or it can be done for one treatment occasion including all projections using a body phantom with detectors. Both methods have implementations using traditional measurement techniques and both of them have important limitations both in methodology and in measurement accuracy.
The shortcoming of the first method is:                Complicated and time-consuming to verify each projection individually rather than the total contribution from all projections in one comparison.        Unnecessary efforts invested in correcting minor errors per projection that would have shown to be neglectable if all projections could be totalled.        The verification excludes errors in gantry angle and collimator rotation since the device is either attached to the gantry or the gantry rotation is not used during verification.        It is not useful in ARC-therapy (described above).        Lack of time resolution in the measurements disables the possibility to analyse the course of a measured deviation, e.g. in sub fields or segments of a field without updated measurement. Additionally there is no possibility to distinguish whether a dose is delivered when expected during the respiratory cycle.        
The first method has been implemented in a product, MapCheck available from Sun Nuclear INC., consisting of a matrix of diodes where each detector integrates the dose during the delivery of one projection. Measuring at one depth in the same beam direction simplifies most of the requirements on the detector to similar to those in traditional measurements. However the requirement to measure with high accuracy outside the primary field, described above, raises several demands on the detectors, and one of the hardest to fulfil for semiconductors is energy independency.
The second method simulates the patient on the couch using a body shaped or head-and-neck shaped plastic-phantom, see U.S. Pat. No. 6,364,529 [MED TEC IOWA INC (US)), with some kind of detectors inserted into it. The phantom that is placed on the couch, without connection to the gantry rotation, can be irradiated similar to a patient in any relevant projection. Thereby the delivered dose from all projections can be measured at any point inside the phantom. Error in delivery, e.g., in MLC position, gantry angle, collimator rotation etc., will cause similar dose discrepancy in the phantom as in the patient. Until now this method has been used with radiological films placed inside the phantom in the direction of the beam and a few point-detectors. The film measures thereby in 2-dimension (2D) along the beam with high spatial resolution. However, across the field, where the beam is modulated, the method is limited to measure along the film (1D). The main reason for the orientation of the film is the shortcoming of film as a detector. The response of radiological films depends on several parameters e.g. direction of radiation, energy, pressure (the pressure on the film at exposure), development process, fading, linearity etc. Additionally, film is an integrating detector so that the film-data has no time resolution, and thereby, analyses of the cause of a deviation between measurement and the treatment plan often become more or less impossible. Ideal point detectors would measure the point-dose accurately; however, a few point detectors will not enable verification of the intensity modulated beam in the various projections.
Ideal detectors do not exist and currently used measuring methods that have no time resolution and/or synchronisation or documentation to the treatment phase makes it impossible to apply relevant corrections to the measurement and thereby improve the result.
Direct measuring detectors currently used on the radiotherapy market are ionisation chambers and semiconductors. The ionisation chambers have in general a better long term stability than semiconductors. However, the spatial resolution of the ionisation chamber is rather limited, normally about 3-4 mm, which is a major limitation in the applications discussed herein.
Nearly 10 years ago scintillation detectors were proposed for radiation therapy, but is has however not been possible to make this technology work in practice. One of the main reasons is that the PM tube or photodiode that is used to convert the light to an electric signal must be kept out of the primary beam and the fibre optics used to connect the scintillation detectors to the PM tube or the photodiode creates scintillations as well. Proposals using dummy fibre optics has been presented, but the underlying technical problems has not been possible to solve.
Semiconductors are mainly diodes or MOSFET detectors. Both these types are based on silicon and thereby they have the same or similar energy dependency. Both have a high specific efficiency measuring radiation, which is an important parameter when measuring small doses. In “Investigation of the use of MOSFET for clinical IMRT dosimetric verification,” Chuang, F. Cynthia, et. al., Med. Phys. 29(6), June 2002, a MOSFET detector system for IMRT verification is disclosed. This system provides for an easy calibration and an instantaneous read-out of test results but shows reproducibility, linearity, energy and angular responses similar to that of conventional dosimeters. The major drawback with the disclosed MOSFET detector system is however the limited lifespan of the detectors, which is mainly caused by radiation damage. Normally, the tolerance against radiation damage is approximately 200 Gy for a MOSFET detector. Moreover, the absorbed dose in a MOSFET can be read directly or after use, but not in real time applications.
Diodes are very reliable detectors with a high tolerance against radiation damage that exceeds 200000 Gy, i.e. approximately 1000 times higher in comparison with a MOSFET. Both MOSFET detectors and ionisation chambers require a bias, which complicates a system with an extensive number of detectors. Diodes are generally very reliable detectors and are used in many applications, e.g., integrating measurements as in vivo dosimetry and output factor measurements in small fields respectively real-time measurement, e.g., relative measurement in water phantoms. The main limitation is the energy dependence and the long term stability even though the latter has been improved during the recent years.